The use of spraying technology for paper coating and sizing applications was tested in the early seventies. This was followed by efforts to apply this technology in a production environment, but the results were largely unsuccessful. The paper industry has been slow to embrace this process, citing potential efficiency impairments created by interruptions in continuous operations. Any such interruption is of great concern in this very capital-intensive industry where production plants must operate non-stop 24 hours a day, seven days a week to remain competitive.
Using spraying techniques to coat or size paper is, in principle, very simple, i.e., a set of nozzles in an applicator box to spray size or coating fluid. This process is shown in minute detail in U.S. Pat. No. 4,944,960 by Donnelly, Kangas and Sundholm, and further developed in their European patent EP 0682571. There have been further efforts to develop spray coating based on high pressure, small-opening nozzles operating at or above 100 bar pressure level, as shown by Koskinen et al. in U.S. Pat. No. 6,060,449.
While spraying with nozzles is certainly not a new process, creating a fine evenly distributed, controllable spray pattern was largely unachievable before the approach outlined by Winheim in U.S. Pat. No. 4,946,101. His patent outlines the historical development of spraying technology in detail, and also introduces the provision for adding a second gas stream via an outer nozzle. This implementation results in an enhanced dispersion capability, but lacked a thermal barrier and the ability to use lubricants and coolants.
When air is used to spray highly viscous and fast-solidifying liquids containing a high concentration of organic and inorganic solids and chemicals, some of these sprayed liquids will crystallize or solidify rather quickly onto the nozzle outlet areas such as the nozzle tip or the outside area from where the dispersing air is released. After the deposit is formed, the spray pattern will be distorted and the process must be stopped to clean the nozzle. This is not acceptable for the paper industry, as mentioned earlier.
Trouble-free spraying has been developed by this invention by preventing the viscous material from solidifying anywhere in the spray nozzle. This improvement to the current technology results from three factors:                1. The nozzle and nozzle tip design as explained in this invention        2. Use of surfacing materials or nozzles made of heat barrier materials including various polymers.        3. Rendering the sticky material or liquid components harmless by preventing contact through lubrication or by dissolving the problematic liquid components (e.g., starch, clay, latexes etc.)        